In the cable industry, it is well known that changes in ambient conditions lead to differences in vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level inside the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. High levels of condensed moisture inside a cable sheath system may have a detrimental effect on the transmission characteristics of a metallic conductor cable.
Furthermore, water may enter the cable because of damage to the cable which compromises its integrity. For example, rodent attacks or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to enter, and, if not controlled, to move longitudinally along the cable into splice closures, for example.
Lately, optical fiber cables have made great inroads into the communications cable market. Although the presence of water itself within an optical fiber cable is not necessarily detrimental to its performance, passage of the water along the cable interior to connection points or terminals or associated equipment inside closures, for example, may cause problems especially in freezing environments and should be prevented.
Cables for transmitting communications signals must meet industry standards with respect to water blocking provisions. For example, one industry standard requires that there be no transmission of water under a pressure head of one meter in one hour through a one meter length of cable.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and along the core. For example, a metallic shield which often times is used to protect a metallic conductor cable against lightning and rodent attacks is provided with a sealed longitudinal seam. Forming of a shield about a cable core requires the use of relatively low manufacturing line speeds. Also, the use of a metallic shield is destructive of the otherwise all-dielectric property of an optical fiber cable.
Because lightning strikes may cause holes in a metallic shield, it is not uncommon to include additional provisions for preventing the ingress of water into the core. Water blocking materials have been used to fill cable cores and to coat portions of cable sheath systems to prevent the movement longitudinally thereof of any water which enters the cable. Although the use of a filling material, in the form of a grease, causes housekeeping problems, inhibits line speeds because of the need to fill carefully interstices of the cable core and presents problems for field personnel during splicing operations, for example, it continues to be used to prevent entry of the water into the core.
Presently, many commercially available cables also include a water swellable tape. The tape is used to prevent the travel of water through the sheath system as well as its travel longitudinally along the cable to closures and termination points, for example. Such a tape generally is laminated, including a water swellable powder which is trapped between two non-woven tissues. Although such a tape provides suitable water protection for the cable, it is relatively expensive and thick. If the tape is too thick, the diameter of the cable is increased, thereby causing problems in terminating the cable with standard size hardware.
The problem of cable size caused by bulky tapes has been overcome. In U.S. patent application Ser. No. 115,123 which was filed on Oct. 30, 1987 in the name of C. J. Arroyo, U.S. Pat. No. 4,867,526, a cable having water blocking provisions is disclosed. Interposed between a core and a jacket is an elongated substrate member which comprises an impregnated non-metallic, non-woven, web-like material in the form of a tape. The tape material is relatively compressible and has sufficient porosity to permit entry of sufficient impregnating material so that it provides enhanced water blocking capability. The impregnating material may comprise a film of a water swelling or so-called superabsorbent material.
In another prior art cable, a water blocking yarn is interposed between a core tube and an outer surface of a jacket of the cable's sheath system. The yarn extends linearly along the cable or may be wrapped helically about a portion of the sheath system. The yarn may be one which is composed of a superabsorbent fiber material which upon contact with water swells and inhibits the movement of water within the cable.
Although the foregoing arrangements provide excellent water blocking capabilities, they generally have been used to supplement a composition of matter such as a jelly-like grease, for example, which fills the core. These compositions of matter are somewhat messy to apply and require a cleaning agent such as a solvent to remove the filling compound to facilitate splicing. Also, care must be taken so that these agents do not affect adversely coloring material or coating material on the fiber. What is sought after is a cable in which the core is not filled with a grease-like material but rather includes other provisions for blocking water flow along the core.
The prior art also includes a cable in which the core is water blocked without resort to jelly-like filling materials. For example, disclosed in an article from the Sumitomo Electric Technical Review is a centrally disposed strength member which has a plurality of optical fibers arrayed thereabout. Water blocking string is disposed in interstices between the fibers and the strength member and a water blocking yarn is disposed between the fibers and a water blocking tape disposed in engagement with a core tube. Also in a brochure of Lopp-Textrina AG, which appears to have a publication date of Jan. 1, 1988, a communication cable includes copper wires having a centrally disposed water swelling yarn and a swelling non-woven is disposed between the copper conductors and a casing. The foregoing alternatives to a core filled with a filling compound may not be the most cost-effective, may not result in optimum space efficiency and may be restricted in their application to particular cable designs such as stranded cable.
Another consideration in the design of optical fiber cables is one which relates to coupling of the optical fibers within the core to the cable sheath system. Of course, the fiber should be suitably coupled in a longitudinal direction to the cable sheath system so that when pulling forces are imparted to the cable, the fibers except perhaps along a relatively short end portion will move with the sheath system. However, in order to avoid unacceptably high microbending losses and/or to mitigate stresses, the optical fibers should be decoupled from the sheath system in at least one direction transverse of the cable.
What the prior art appears to lack is a cable in which a core of the cable is provided with a simple water blocking arrangement instead of multiple provisions which require additional manufacturing steps and expense. Such a sought-after arrangement should be one in which the optical fibers are decoupled substantially from the sheath system in at least one direction transverse of the cable.